For analysis of the flue gas of furnace installations, the flue gas is sucked in by means of a probe and fed to an analysis apparatus. The flue gas must be treated prior to analysis thereof, i.e., it is necessary in particular to remove smoke and dirt particles as well as moisture from the flue gas.
The flue gas is passed from the probe through a tube to the analysis apparatus. If condensate is deposited in the tube, the flue gas in the tube comes into contact with the liquid condensate over a long period of time. In the process, NO.sub.2 and SO.sub.2 are partly fixed in the liquid condensate, resulting in measurement errors.
It is known that such measurement errors can be prevented by using a heated tube. In the tube, the flue gas is heated to a temperature above the dew point, with the result that condensate is not deposited. The entire deposition of condensate takes place in a cooled condensate trap incorporated upstream from the analysis apparatus. Thereby, the condensate is collected rapidly and within a short section of the gas-flow path, and so the duration of contact of the gas with the liquid condensate is limited and NO.sub.2 and SO.sub.2 are fixed to only a slight extent, with a nonsignificant falsifying influence on the measured result.
It is known that a filter can be interposed between the flue-gas probe and the heatable tube for the purpose of removing dirt and soot particles from the flue gas being fed to the analysis apparatus. A disadvantage of this expedient is that condensate can be deposited in this filter. This leads to the aforesaid falsification of the measured result, since NO.sub.2 and SO.sub.2 become partly fixed in the condensate. In addition, the moisture of the condensate in the filter leads to more rapid fouling of the filter by dirt and smoke particles.
To circumvent this problem, it has been suggested in West German Laid-open Application 1,949,081 that the filter be equipped with an electrical heating device. This is located inside a housing, and is constructed, for example, in the form of a helical heating coil around the filter element, which is also located in the housing. This makes it possible to keep the temperature of the flue gas above the dew point. The disadvantage of such a configuration lies in the fact that the heatable tube and the upstream heatable filter are provided as separate units, each with its own heating system. The necessary expense is high; problems occur in particular with respect to dimensions and weight, if the analysis of the flue gas is to be undertaken not by fixed units but instead by mobile units.
The object of the invention was therefore to provide, for a flue-gas analysis apparatus, a gas-sampling means that does not have the described disadvantages. As regards the possibility of mobile use, a particular object is to provide a gas-sampling means of light weight and small dimension.